Chamfered freestanding nitride semiconductor wafer and method of chamfering nitride semiconductor wafer

ABSTRACT

Technology of making freestanding gallium nitride (GaN) wafers has been matured at length. Gallium nitride is rigid but fragile. Chamfering of a periphery of a GaN wafer is difficult. At present edges are chamfered by a rotary whetstone of gross granules with weak pressure. Minimum roughness of the chamfered edges is still about Ra 10 μm to Ra 6 μm. The large edge roughness causes scratches, cracks, splits or breaks in transferring process or wafer process. A wafer of the present invention is bevelled by fixing the wafer to a chuck of a rotor, bringing an edge of the wafer into contact with an elastic whetting material having a soft matrix and granules implanted on the soft matrix, rotating the wafer and feeding the whetting material. Favorably, several times of chamfering edges by changing the whetting materials of smaller granules are given to the wafer. The chamfering can realize small roughness of Ra 10 nm and Ra 5 μm at edges of wafers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improvement of an edge (periphery) of anitride semiconductor single crystal circular wafer. The edge means aperiphery of a wafer. An as-cut wafer has a sharp edge which causesbreak, crack, split, chip or scratch of the wafer. Then, the edge partis slantingly polished into a round periphery. The slanting polishing ofedges is called “bevelling” or “chamfering” for discriminating the edgeslanting polishing from flat polishing of wafer surfaces. Semiconductornitrides mean gallium nitride (GaN), indium nitride (InN) and aluminumnitride (AlN). The nitrides are rigid but fragile materials. Processingof semiconductor nitride wafers is far more difficult than silicon (Si)wafers or gallium arsenide (GaAs) wafers.

This application claims the priority of Japanese Patent Application No.2003-98979 filed on Apr. 2, 2003 and Japanese Patent Application No.2003-275935 filed on Jul. 17, 2003, which are incorporated herein byreference.

It has been very difficult to produce a large good semiconductor nitridebulk single crystal wafer. Someone has recently succeeded in producingfreestanding GaN wafers on a small scale. Most of the freestanding GaNwafers are still small rectangular plates whose side is about 10 mm to20 mm. Technology of producing GaN wafers is still not fully matured forserving GaN crystals as a substrate wafer of making InGaN type bluelight lasers on a mass-production scale. Circular single crystal wafersof InN and AlN have not been yet produced except experimental trials.

At length, the production of circular GaN single crystal wafers becomesfeasible. An unprocessed sharp edge of an as-cut wafer causes cracks,scratches or breaks of the wafer. Peripheral parts (edges) ofsemiconductor wafers are used to be slantingly polished for avoidingoccurrence of cracks or breaks. The process of polishing waferperipheries into slanting edges is called “bevelling” or “chamfering”.Bevelling or chamfering has been one of established processes in thecase of silicon wafers or gallium nitride wafers for which productiontechnology has been fully matured. Silicon wafers are rigid, sturdy andtough. A silicon wafer is chamfered by bringing a periphery of the waferinto contact with a rotary whetstone, rotating both the whetstone andthe wafer in reverse directions and polishing the periphery into aslanting, round edge. Toughness of silicon enables rotary whetstones tochamfer edges of silicon wafers.

2. Description of Related Art

{circumflex over (1)} Japanese Patent Laying Open No. 9-181021,“Beveling method of wafer”, proposed a method of bevelling a siliconwafer by a sophisticated diamond rotary whetstone including 5 wt. % to30 wt. % of ultrafine diamond particles of a diameter from 3 nm to 18 nmand 70 wt. % to 95 wt. % of small sized diamond granules of a diameterfrom 5 μm (5000 nm) to 8 μm (8000 nm). This is a complex rotarywhetstone including two different kinds of whetting granules, one is theultrafine particles and the other is the small particles. {circumflexover (1)} wrote that the conventional rotary whetstones had beencomposed of gross granules of an average diameter of 15 μm to 30 μm forchamfering silicon wafers and the gross granules caused breaks of wafersor cracks on wafers. For avoiding the beaks and cracks, {circumflex over(1)} proposed silicon wafer bevelling by the fine particle whetstone.Use of the ultrafine particles of a diameter of 3 nm to 18 nm suppressescracks or breaks from occurring. The small sized particles of a diameterof 5 μm to 8 μm were intentionally used for compensating for delay ofchamfering induced by the ultrafine particles. {circumflex over (1)} wasan improvement of bevelling silicon wafers by a fine particle implantedrotary whetstone for avoiding occurrence of breaks and cracks.{circumflex over (2)} Japanese Patent Laying Open No. 6-315830,“Beveling method for cut-resistant material”, proposed an electrolyticbevelling method for silicon wafers. {circumflex over (2)} complainedthat silicon wafers were too rigid and resistant to mechanically beveland diamond whetstones should be used for silicon bevelling which raisedthe cost of silicon chamfering. The electrolysis method of {circumflexover (2)} supplies silicon wafers with an electrolyte, applies voltageto the silicon wafer via the electrolyte and chamfers edges of thewafers by the action of electrolysis. {circumflex over (1)} and{circumflex over (2)} are improvements of silicon wafer bevelling.{circumflex over (3)} Japanese Patent Laying Open No. 2002-356398 (Dateof publication of application: Dec. 13, 2002), “Gallium nitride wafer”,proposed an invention of bevelling of GaN wafers by the same inventionas the present invention.

Current chamfering technology of the inventors of the present inventionhas chamfered a circular GaN wafer by making use of a rotarymetal-bonded circular whetstone having implanted diamond granules of#100 mesh to #400 mesh (optimum mesh; #200), circumscribing a circularwafer to the rotary whetstone, and rotating the rotary whetstone and thewafer in inverse directions at 800 m/min to 2000 m/min which is anordinary range of rotation speed, as shown in FIG. 1. A periphery 6 ofthe circular wafer 2 is pushed to a concave rotary whetstone 3 whichwears fixed diamond granules on an outer round surface. The wafer 2 iscircumscribed with the rotary whetstone 3. A liquid is supplied to therotary whetstone 3 and the wafer 2. The rotary whetstone 3 rotates at ahigh speed in a direction. The wafer is also rotated in an inversedirection at a low speed. The implanted granules abrade the periphery ofthe wafer at a high rate. The circumscribing contact applies strongshocks upon the edge of the wafer. It takes about ten minutes to twentyminutes to eliminate a diametrical 1 mm wide margin. It is rapidbevelling. A final shape of the edges is determined in accordance withthe standard of the SEMI (Semiconductor Equipment and MaterialsInternational).

FIG. 5 depicts sizes of parts of a 2-inch GaN wafer in the chamferingsteps. A necessary bevelling margin is a radial 1 mm width. An initialwafer has a 52 mm diameter and a 520 μm thickness. Before chamfering, anorientation flat OF and an identification flat IF are formed atperipheral portions by a dicer or a grinding whetstone for signifyingcrystallographical orientations. A final length of the OF should be 16mm. A length of the IF should be 7 mm. The OF line should be cut along aline distanced by 2.32 mm from a point on the circumference. The OF isusually a cleavage plane {1-100}. The IF line should be cut alonganother line distanced by 1.25 mm at another point on the circumference.The IF is vertical to the OF. Relative positions of the OF and the IFare determined for clockwise aligning in the order when a facing surfaceis a top surface. Sizes of parts are predetermined as such.

Prior edge processing of the inventors of the present inventionchamfers, for example, a 52 mmφ GaN wafer having OF and IF bycircumscribing the wafer with a metal-bonded diamond rotating whetstone,rotating the diamond whetstone, grinding a circumference of the wafertill the outer diameter of the GaN wafer is reduced to 50 mmφ, as shownin FIG. 1. The diameter should be reduced by 2 mm. The periphery whichis eliminated away is called a polishing margin. The polishing margin is1 mm in radius or identically 2 mm in diameter. Hard diamond fixedgranules have a strong force of abrading rugged wafer edges. Suchbevelling for the diametrical 2 mm margin takes only about 20 minutes to40 minutes. The processing time is short enough. The processing improvesroughness of the wafer down to about Ra10000 nm to Ra6000 nm (Ra10 μm toRa6 μm) in the best case. However, the yield of chamfering is low.Sometimes the periphery is scratched or chipped in the chamfering step.Sometimes the wafer cracks or breaks. Impulsive contact with the hardrotary whetstone is often destructive for the wafer. Ra6 μm is thelowest limit in the circumscribing rotary contact whetstone. It isdesired to reduce the roughness less than Ra5 μm. But, such smoothnessless than Ra5 μm is beyond the power of the metal-bonded circumscribingrotary whetstone rotating at an ordinary speed. It may supposed that avery slow rotation whetstone will be able to bevel the fragile GaN waferwithout scratching, chipping or cracking. A ten-hour whetting making useof the same metal-bonded diamond rotary whetstone succeeded inchamfering a circumference to roughness of about Ra3 μm. However, theslow whetting has drawbacks of excess dissipation of whetstones, jammingof meshes with dust and big fluctuation of final properties of thewafer. The long-time whetting is not practical for giving mirrorsmoothness to GaN wafers.

Whetting powder of #200 is far coarser powder than the whetting powderfor polishing the silicon wafers described in the previously cited{circumflex over (1)} Japanese Patent Laying Open No. 9-181021,“Beveling method of wafer”. Thus, a wafer polished by the #200 powderhas a rough surface of Ra10 μm to Ra6 μm.

The inventors had employed the coarse whetting powder for bevelling GaNwafers. GaN wafers are rigid but fragile. The rigidity forced theinventors to employ the coarse powder. It is, however, difficult tofinish GaN wafers due to the fragility. If a GaN wafer is inouter-contact with a rotating whetstone and the edge is bevelled by therotating whetstone, the edge or the whole of the wafer breaks during thebevelling process with high frequency. Even if the edges are not broken,the chamfered edges suffers from high roughness of Ra=10 μm to 6 μm asdescribed before.

It may be perhaps effective to use rotating whetstones of fine granulesof smaller diameters for reducing the occurrence of scratches. However,employment of smaller-sized whetting granules requires longer time forpolishing edges of wafers, which raises cost of edge polishing. Use ofthe fine granule whetstones has other drawbacks of raising theprobability of cramming meshes with whetting wastes and shortening thelifetime of whetstones.

One purpose of the present invention is to provide a freestandinggallium nitride wafer which is free from occurrence of cracks fromperipheral parts (edges).

Another purpose of the present invention is to provide a freestandinggallium nitride (GaN) wafer which is free from occurrence of scratchesor breaks from edges.

Another purpose of the present invention is to provide a freestandinggallium nitride (GaN) wafer with clean edges which do not inviteparticle adhesion or waste contamination. A further purpose of thepresent invention is to provide a method of chamfering an edge of anitride wafer without incurring scratching, splitting or breaking of thewafer. A further purpose of the present invention is to provide a methodof chamfering an edge of a nitride wafer without clogging of meshes withwhetting materials. This invention is also applicable to other nitridesemiconductor wafers of aluminum nitride (AlN) or indium nitride (InN)besides GaN.

SUMMARY OF THE INVENTION

The present invention proposes a nitride semiconductor wafer having anedge bevelled to roughness Ra smaller than Ra5 μm but larger than Ra10nm (10 nm≦Ra≦5 μm). The roughness less than Ra5 μm enables the nitridewafer to reduce a crack occurrence rate under 50%.

More favorably, the present invention proposes a nitride semiconductorwafer with a smooth edge bevelled to roughness Ra smaller than Ra1 μmbut larger than Ra10 nm (10 nm≦Ra≦1 μm). The edge roughness less thanRa1 μm allows the wafer to decrease a crack occurrence rate under 10%.

Furthermore, the present invention proposes a edge roughness less thanRa0.1 μm for a nitride semiconductor wafer. The small roughness lessthan Ra0.1 μm can suppress a crack occurrence rate under 6%.

In any cases, the lowest limit of roughness is Ra10 nm. Ra10 nm is thelimit restricted by the method of the present invention relying uponwhettapes. Ra10 nm is equal to a value in a scope of minute flat surfacepolishing. The edges need not have extremely high flatness equivalent tothe flatness of surfaces.

Thus, the present invention proposes a nitride wafer having edgeroughness of,

(1) Ra5 μm˜Ra10 nm,(2) Ra1 μm˜Ra10 nm or(3) Ra0.1 μm˜Ra10 nm.

FIG. 6 shows a section of a GaN wafer having an edge which is bevelledby an apparatus of the present invention. The edge is polished into around shape. The polished edge can have flat portions instead of theround portions. Polishing of the edge portion is done by a whettingmaterial which has a flexible body and continuously supply a cuttingedge to an object wafer, for example, a whettape. The tape-polishing canrealize a natural round shape of the edges.

Smoothing the edge of wafers reduces an occurrence rate of cracks andenhances yield of producing good wafers. The smooth edges also decreasethe probability of breaks and cracks on wafer processes.

Rotation whetstones, however, are not suitable for polishing edges ofGaN wafers, because GaN is fragile. The present invention employswhettapes instead of the rigid rotation whetstones. Whettapes mean acontinual coiled tape paper or cloth implanted with polishing granules5. FIG. 2 and FIG. 3 show a polishing process by bringing an edge 6 of awafer 2 into contact with a whettape 4, rotating the wafer 2 andpolishing the edge 6 of the wafer 2. The whettape 4 is soft enough tobend into arbitrary curvatures, because a base is paper or cloth whichis rich in elasticity.

Besides, a contacting area between a wafer and a whetting materialdiffers. A conventional rotating whetstone is in outer-contact with anedge of an object wafer. The contact is a spot contact in theconventional wafer/whetstone. An effective area of the contact is verynarrow. The contacting pressure per unit area is quite strong. Thestrong pressure is apt to break wafers in the conventional rotatingwhetting.

The whettape shown in FIG. 3, however, has a larger effective contactingarea indicated by points EFG with the wafer, because the whettapeflexibly bends along the wafer. As shown in FIG. 3, the wafer 2 isinscribed in the whettape 4 at the points E, F and G. Since the whettape4 has elasticity, a central angle EOG of the wafer 2, which shows acontacting area between the wafer and the whettape, can be 40 degrees to90 degrees. The contacting area is wide along a circumference of thewafer as illustrated in FIG. 3 and is also wide along a thickness of thewafer as shown in FIG. 2. Therefore, since the contacting area is widein the case of the whettape, a contacting pressure per unit area can befar smaller than that of the conventional whetstone. Probability ofoccurrence of breaks of wafers in process can be effectively suppressed.

Probability of occurrence of clogging is higher in finer whetstones thanin grosser whetstones in the case of rotary whetstones. Unlike therotary whetstones, the whettapes always renew whetting granules 5implanted on a continual cloth tape at a feeding speed U. The continualrenewal of acting granules prevents whetdust from remaining and fromjamming into gaps of granules. Due to the continual replacement ofgranules, the whettape is immune from the occurrence of clogging ofwhetdust. Suppression of clogging enables the whettape to make use offar finer granules than the ordinary rotary whetstone.

Advantages of the present invention are described. The inventionproposes a freestanding nitride wafer having an edge of high smoothnessof Ra10 nm to Ra5 μm and a method of chamfering a circular freestandingnitride wafer by a whettape having implanted granules. The whettape isrich in elasticity, softness, shock-absorption and toughness. Thewhettape inscribes an edge of a wafer, which increases a contact lengthand reduces the force per unit area. Such features of the whettapereduce the probability of splitting, scratching or breaking inchamfering steps and enhance the final obtainable edge smoothness toRa10 nm to Ra100 nm. Feeding of the whettape renews contacting granulesand prevents the fixed granules from being jammed with waste.

Prior bevelling by the rotary whetstones can chamfer the edges ofcircular wafers to Ra10 μm to Ra6 μm at best. It is impossible to obtainsmoothness under Ra6 μm by the prior rotary whetstone method. As shownin FIG. 4, the roughness of Ra10 μm to Ra6 μm incurs over 60% of a crackoccurrence rate. An upbeat of edge smoothness enables the presentinvention to reduce the probability of occurrence of cracks and raisethe yield of bevelling. FIG. 4 indicates Ra5 μm, which corresponds tothe upper limit of the present invention and invites cracks at a 50%rate. This invention can produce good wafers with a crack occurrencerate less than 50%.

Preferable smoothness is Ra1 μm to Ra10 nm. The roughness under Ra1 μmensures the present invention to reduce the crack occurrence rate toless than 10%.

Further favorable smoothness is Ra0.1 μm to Ra10 nm. The roughness underRa100 nm decreases the crack occurrence probability under 6%.

In addition to the cracks, the present invention can decrease theprobability of breaks, splits, crashes and contamination by waste, dustor foreign materials in wafer processes and in conveying processesthrough the enhanced smoothness of wafer edges. GaN, AlN and InNfreestanding wafers of high quality are obtained by the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a spool-shaped rotating whetstone with aninner curved polishing surface for bevelling edges of a wafer forillustrating a conventional wafer bevelling method.

FIG. 2 is a vertically sectioned view of a part of a freestanding GaNwafer and an elastic whettape being in contact with an edge of the GaNwafer and running in an angular direction around the edge for showing awafer bevelling method of the present invention.

FIG. 3 is a horizontally sectioned view of a part of the freestandingGaN wafer and the elastic whettape being in contact with the edge of theGaN wafer and running in an angular direction around the edge forshowing a wafer bevelling method of the present invention.

FIG. 4 is a graph for showing a relation between a surface roughness Ra(μm) of edges of GaN wafers and an occurrence rate (%) of cracks. Theabscissa is the surface roughness Ra (μm) of edges. The ordinate is thecrack occurrence rate (%).

FIG. 5 is a graph showing sizes of an original free-standing 2-inch φGaN wafer which has a 52 mm diameter and a 520 μm thickness, and aprocessed wafer having an orientation flat (OF), an identification flat(IF) made on edges of the wafer by edge grinding and an edge which ispolished and reduced by 2 mm diameter (1 mm radius) by a whettape inaccordance with the teaching of the present invention.

FIG. 6 is a sectional view of a GaN wafer which has an edge polished bya whettape into a predetermined surface roughness.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention proposes a chamfering method of bringing an edgeof a circular nitride wafer to a whettape, rotating the nitride wafer incontact with the whettape, feeding the whettape and polishing the edgeof the wafer to roughness Ra between Ra5000 nm and Ra10 nm, morefavorable roughness between Ra1000 nm and 10 nm and the best caseroughness between Ra100 nm and Ra10 nm. This invention further proposesa nitride wafer having a chamfered edge of roughness Ra of

(1) Ra5000 nm to Ra10 nm,

more favorable edge roughness of

(2) Ra1000 nm to Ra10 nm,

and the best edge roughness of

(3) Ra100 nm to Ra10 nm.

The whettape method of the present invention polishes a periphery of aGaN (or InN or AlN) wafer by bringing an edge of the wafer in contactwith a whettape having implanted granules, rotating the GaN wafer in theenclosing whettape and feeding the whettape at a slow speed. Unlike therotating whetstone, the whettape polishing of the present inventionrotates the wafer. The powder granules are implanted on a soft, elasticcloth or paper. The whettape can be made of cloths, polyurethane,leather, rubber or paper. Elastic and soft contact with the whettapeabsorbs shocks and alleviates the force acting upon the wafer from awhetting machine. The whetting granules are, for example, siliconcarbide (SiC), alumina (Al₂O₃), diamond (C) or silica.

And, a liquid for whetting can be also used with the granules and theliquid includes silicon carbide (SiC), alumina (Al₂O₃), diamond (C) orcolloidal silica. The tape for whetting is made of cloths, polyurethane,leather, rubber, or paper. Since the whettape is in inscribing contactwith the edge, a contacting region is still wider than the case of theordinary rotary whetstone which circumscribes a wafer. The wide contactarea can reduce pressure (force per unit area) acting between thewhetting matter and the wafer. The favorable pressure is 1 kg/cm² to 10kg/cm². Continual renewal (feeding) of the whettape inhibits the meshfrom jamming with dust. Soft touch of powder, the shock-absorbing tape,the inscribing contact, slow rotation and long time processing canafford to give edges of wafers mirror-smoothness without incurringsplits, scratches, breaks and cracks.

Adjustable variables are sizes (average diameter or mesh number #) ofgranules implanted upon the whettape, a contacting force F between thewhettape and wafer edge, a line velocity V (angular velocity×radius), apolishing time H, a tape feeding speed U and an abrasive liquid. Asingle-step polishing, a two-step polishing, a three-step polishing andan over-three step polishing are available for the whettape method. Inthe case of multiple-steps, tapes having larger granules are followed bytapes having smaller granules. A larger mesh number (#) means a smalleraverage diameter of granules.

Mesh sizes of granules applied to a pertinent whettape should be #300 to#5000, more favorably, #500 to #3000. Suitable feeding speeds of thewhettape which depend upon granule mesh sizes are 5 mm/min to 60 mm/min,more favorably, 10 mm/min to 30 mm/min. A whettape implanted with largergranules is more powerful to abrade edges but is unable to producehigh-smoothness of the edge. Another whettape implanted with smallergranules is more subject to suffer from abrasion wearing, which shouldrequire higher speed of feeding. A single step of a single whettape isavailable. A plurality of steps with different mesh whettapes are alsoeffective. For example, a first step employs a whettape of #300 to#1000, a second step employs another whettape of #1000 to #2500 and athird step employs another whettape of #2500 to #5000. Unlike theordinary rotary whetstones, current whetting portions of the tape can becontinually renewed with the feeding of the whetting tape. Thecontinuous renewal of whetting cloth parts prevents implanted smallgranules from jamming with waste and maintains good polishingperformance.

Since the tapewhetting is substantially a kind of mechanical polishing,a suitable polishing liquid is, for example, powder including water,powderless water, powder including oil and powderless oil. The polishingliquid has functions of cooling the contacting portions, reducingabrasion resistance, alleviating shocks acting upon edges of the objectwafer and raising yield of chamfering. Necessary polishing time dependsupon sizes of granules implanted on the tape. Gross-chamfering makinguse of larger granule tapes consumes short time. Minute chamferingprocessed by smaller granule tapes requires long time.

The processing time H varies at the steps of chamfering. Each steprequires one hour to ten hours. The smaller the granules are, the longertime the bevelling step requires. The present invention has a drawbackof requiring longer processing time than the conventional rotarywhetstones which take only twenty minutes to forty minutes. Thetime-consuming drawback of the present invention can be compensated byan improvement of roughness Ra of edges of wafers.

About seven hour polishing reduces the wafer diameter by about 2 mm.About 13 m of a whettape is consumed for edge-polishing of one wafer atthe polishing rate. Neither cracks nor splits occur. Final roughness ofthe edges is Ra10 nm to Ra5 μm. More favorable cases obtain finalroughness between Ra10 nm and Ra1 μm. The most favorable case enjoysfinal roughness between Ra10 nm and Ra100 nm. The length of whettape perwafer would be able to be decreased down to about 10 m. This inventioncan be applied to other nitride semiconductor wafers, aluminum nitride(AlN) wafers and indium nitride (InN) wafers.

Embodiment

A starting wafer is an as-grown GaN wafer of a 52 mm diameter and a 520μm thickness. An orientation flat (OF, 16 mm of a final length) and anidentification flat (IF, 7 mm of a final length) are formed on sides ofthe GaN wafer by, for example, a dicer. FIG. 5 shows a plan view of theobject GaN wafer for showing sizes of the wafer. A 52 mmφ outer fullcircle denotes an original size of the object wafer beforeedge-polishing. A left straight line shows a cutting line for the IFformation. A bottom straight line denotes a cutting line for the OFformation. A 1 mm wide edge part is eliminated by the edge polishing. A50 mmφ inner circle shows the edge-polished wafer. Since a 1 mm wideperipheral circular part is eliminated, the lengths of the left IF lineand the bottom OF line are reduced to the predetermined lengths of 7 mmand 16 mm.

The original 52 mmφ wafer is fixed to a chuck of a rotor by aligningcenters. At a first bevelling step, an endless gross whettape of #800mesh is used for bevelling an edge of the GaN wafer. The edge of thewafer is brought into contact with the whettape with a pressure of 7kg/cm². The wafer is rotated at a suitable speed in the state of contactwith the whettape. The whettape is fed at a feeding speed of U=10mm/min. Water is supplied as a coolant to the polished wafer and thewhettape. The wafer edge is chamfered for two hours by the #800 meshwhettape. The roughness is measured by an AFM (atomic force microscope).The first chamfering step using the #800 mesh whettape reduces roughnessof the edge to Ra0.9 μm (=Ra900 nm).

At a second step, another middle whettape of #2000 mesh is utilized. Thegross-polished edge of the wafer is further chamfered by the #2000 meshwhettape at a 20 mm/min feeding speed (U=20 mm/min) with a pertinentpressure. Five-hour bevelling gives the edge a roughness of Ra0.3 μm(=Ra300 nm) by measurement on the AFM.

At a third step, a fine whettape of #3000 mesh is further utilized. Thewafer edge is further bevelled for 6.5 hours by the whettape which isfed at a feeding speed of U=30 mm/min. The roughness of the edge isreduced down to Ra0.1 μm=Ra100 nm by the AFM measurement. A series ofthe three chamfering steps by the whettapes succeeded in obtaining asmooth edge of roughness Ra0.1 μm.

As described before, the first purpose of the present invention is tosuppress the roughness of edges of GaN wafers or other nitride wafersunder Ra5 μm (Ra≦5 μm). Such roughness has been accomplished by thefirst step (Ra0.9 μm by the whettape of #800 mesh) of bevelling of theembodiment. As clarified in FIG. 4, Ra1 μm corresponds to 10% of thecrack occurrence rate. It is significant to suppress the edge roughnessunder Ra1 μm for the sake of preventing cracks from occurring. It ispossible to stop the bevelling at the first step which brings about theedge roughness Ra0.9 μm.

The second bevelling step heightens smoothness of the edge of the GaNwafer up to Ra300 nm by a finer whettape of #2000. The second step canaccomplish the smoothness of the range (2) (Ra1000 nm to Ra10 nm)above-cited. It is also possible to finish the chamfer at the secondstep.

The third bevelling step of the embodiment enhances smoothness of theedge up to Ra100 nm by the finest whettape of #3000. The third stepchamfer can produce excellent smoothness of the edge, which suppresses acrack occurrence rate to 5% to 8%, as exhibited in FIG. 4. There areother factors inducing cracks. Such reduction of the crack occurrenceprobability is a remarkable effect of the slow tape-bevelling of thepresent invention.

1-20. (canceled)
 21. A chamfered freestanding nitride semiconductorwafer having a peripheral edge having roughness of between Ra0.1 μm andRa5 μm.
 22. The chamfered freestanding nitride semiconductor wafer asclaimed in claim 21, wherein the nitride semiconductor wafer is a GaNwafer.
 23. The chamfered freestanding nitride semiconductor wafer asclaimed in claim 21, wherein the nitride semiconductor wafer is an AlNwafer.
 24. The chamfered freestanding nitride semiconductor wafer asclaimed in claim 21, wherein the nitride semiconductor wafer is an InNwafer.